1. Field of the Invention
This invention relates to a chiral smectic liquid crystal composition. More specifically, it relates to a ferroelectric liquid crystal material having a long helical pitch and yet having a large spontaneous electric polarization and superior response property, obtained by mixing chiral smectic compounds in which the twist sense of helix is right-handed and other chiral smectic compounds in which the twist sense of helix is left-handed, without undesirable loss of spontaneous polarization.
2. Description of the Prior Art
Liquid crystals are now being used as display materials to an increasing extent but most of such liquid crystal display elements are of Twisted Nematic (TN) display mode. Since the TN display mode is a non-emissive type, it has characteristic features that it does not give fatigue to eyes and works with extremely small power consumption. On the other hand, it has such a defect as slow response and that display is not observable depending upon the angle from which it is viewed. Recently, attempt to improve liquid crystal material has been made to meet the demand, particularly for the quick response property of display device. However, compared with other emissive displays (e.g. electroluminescense display, plasma display, etc.), there is still a great difference in response time.
To maintain the good response property comparable to emissive displays without losing the characteristic features of non-emissive type as well as its low power consumption, it is indispensable to develop a new liquid crystal display mode which can substitute for a TN type display mode. In one of such attempts, there is a display device which utilizes the light switching phenomenon of a ferroelectric liquid crystal [see N. A. Clark, S. T. Lagerwall Appl. Phys. Lett., 36,899 (1980)]. The existence of a ferroelectric liquid crystal was disclosed for the first time by R. B. Meyer et al in 1975 [see R. B. Meyer et al; J. de Physique 36, L-69 (1975)]. In terms of the usual classification of liquid crystal, this material belongs to chiral smectic C phase (abbreviated as Sc* phase) or chiral smectic H phase (abbreviated as SH* phase).
Application of light switching effect of Sc* phase to display elements has three superior characteristic feature, as compared with TN display mode. The first feature is its very fast response Its response time is less than 1/100 as compared with that of a common TN display mode. The second feature is that it has a memory effect which makes multiplexing drive easy to adopt in co-operation with the above-mentioned quick response. The third feature is the easiness of attaining the gray scale gradation of brightness. In case of a TN display mode gray scale is attained by controlling applied voltage but there are difficult problems such as temperature dependence of threshold voltage, voltage dependence of response time, etc. On the other hand, when light switching effect of Sc* phase is applied, gray scale can be attained easily by controlling polarity-inversion time. Thus the latter is very suitable for graphic display or the like.
As display methods, two methods can be considered. One of them is birefringence type which uses two polarizer plates and the other is guest-host type which uses dichroic dyes. Since Sc* phase are associated with spontaneous electric polarization a molecule turns over by a .pi. rotation around a helical axis which is an axis of rotation by inverting the polarity of applied voltage. By filling a liquid crystal composition having Sc* phase into a liquid crystal display cell that has been subjected to a surface treatment for aligning liquid crystal molecules parallel to electrode surface, and placing the liquid crystal cell between two polarizers arranged in advance so that the polarization plane of one of the polarizers is parallel to the director of liquid crystal molecules and inverting the polarity of applied voltage, the bright range of vision and dark range of vision (which are determined by the angle between the polarization planes) can be inter-exchanged. On the other hand, when operated by guest-host mode, colored range of vision and colorless range of vision (which are determined by the arrangement of polarization plates) can be inter-exchanged by inverting the polarity of applied voltage.
It is considered necessary that Sc* phase having spontaneous electric polarization can take bistable states on the electrode surfaces during the inversion of polarity of applied voltage. To obtain liquid crystal display elements having such bistable states and quick response property, it is necessary to make cell gap d not greater than helical pitch P (d.ltoreq.P) and to unwind the helix as proposed by Clark et al [see N. A. Clark, S. T. Lagerwall Appl. Phys. Lett., 36,899 (1980)].
In general, ferroelectric liquid crystal compounds available now have mostly short helical pitches (about 0.5.about.2 .mu.m) and are not suitable for practical use. Namely, it is necessary to make cell gaps in the range of about 1.about.2 .mu.m to unwind the helix of Sc* phase. With the present cell manufacturing techniques, this presents difficult problems in cost and yield of display. Since the cell gap presently used in TN display mode, is on the order of 7.about.10 .mu.m, it is desired to set helical pitch to 10 .mu.m or greater in order to make ferroelectric liquid crystal displays practicable.
In the attainment of the above-mentioned bistable states of a display element which uses ferroelectric liquid crystal there is, in general, a relation of ##EQU1## wherein E.sub.c is a threshold voltage necessary to invert molecules, P is a helical pitch, Ps is a spontaneous electric polarization and K is a torsional elastic constant at the time of deformation due to turning-over of molecule [see B. Meyer; Mol. Cryst. & Lig. Cryst., 40,33 (1977)]. As evident from the equation (1), to make a threshold voltage smaller, it is necessary that a helical pitch be longer and spontaneous electric polarization be larger. However, in the present, there is no ferroelectric liquid crystal compound which has a long helical pitch and a large spontaneous electric polarization. Almost all of the known ferroelectric liquid crystal compounds have a short helical pitch. Accordingly several kinds of ferroelectric liquid crystal compound must be mixed to make a ferroelectric liquid crystal composition having a long helical pitch and a large spontaneous electric polarization.
It is known that in the case of cholesteric liquid crystals, the twist sense of cholesteric helix depends upon the position of asymmetric carbon atom of a side chain in a compound. It is interpreted that this is due to the alternation of the direction of methyl group attached to an asymmetric carbon atom (C*), depending upon the position of the asymmetric carbon atom. [see: M. Tsukamoto, T. Ohtsuka, K. Morimoto, Y. Murakami; Japan. J. Appl. Phys., 14, 1307 (1975)]. Namely, in case where the position of an asymmetric carbon atom of a side chain is at the even position counting from the carbon atom of benzene ring, the twist sense of helix is right-handed and in case where it is at the odd position, the twist sense of helix is left-handed when the optical active group is chemically derived from the same (S)-2-methylbutanol. We found that an analogous relation exist in the case of twist sense of Sc* phase and position of methyl group at the optical center, which knowledge formed the background of the present invention.
3. Summary of the Invention
The object of the present invention is to provide a ferroelectric liquid crystal composition having a long helical pitch and a large spontaneous electric polarization, which can be practically used in quick response display device by using a cell having a cell gap comparable with the one presently used in TN display mode. To make the helical pitch longer, there is considered a method in which a smectic C compound (abbreviated Sc) which does not have helical structure is added to a ferroelectric liquid crystal compound and also there is considered another method in which a compound having a twist sense of helix right-handed is mixed with another compound having a twist sense of helix left-handed to fully or practically compensate the twist of helix each other.
In the accompanying drawings, FIG. 1 and FIG. 2 show the concentration dependence of spontaneous electric polarization and helical pitch when a Sc compound (B) which does not show helical structure, and is expressed by a following chemical formula ##STR1## is added to a compound A expressed by a following formula ##STR2## As evident from the figures, spontaneous electric polarization becomes smaller as concentration of the compound B is increased and the helical pitch becomes longer. It is easily interpreted that this is due to the reduction of the concentration of the compound A which shows spontaneous electric polarization with the increase of the concentration of the compound B which does not show spontaneous electric polarization, as the result the spontaneous electric polarization as a whole is reduced. Such a method in which a Sc* compound is diluted with a Sc compound to make helical pitch longer, is not considered to be practical, as it is applicable only to the Sc* compound that shows extremely large spontaneous polarization. On the other hand according to the present invention, it is possible to easily obtain a ferroelectric liquid crystal composition having a long helical pitch and a large spontaneous electric polarization by mixing a chiral smectic liquid crystal compound or compounds having a twist sense of helix right-handed with another chiral smectic compound or compounds having a twist sense of helix left-handed. However, the two compounds of opposite twist sense mentioned above should not be the antipodes to each other, because they obviously lead to a racemate in which spontaneous polarization is cancelled out.
Namely, the first aspect of the present invention lies in (1) a ferroeleotric chiral smectic liquid crystal composition comprising, respectively, one or more kinds of chiral smectic liquid crystal compound having a twist sense of helix right-handed, and one or more chiral smectic liquid crystal compound having a twist sense of helix left-handed. Embodiments thereof are as follows. (2) A ferroelectric chiral smectic liquid crystal composition according to the above-mentioned item (1) wherein the chiral smectic liquid crystal compound having twist sense of helix right-handed and not forming a recemate, is expressed by a general formula ##STR3## wherein m or n is an integer of 1 or 2; X is ##STR4## --CH.dbd.H--, --CH.sub.2 O--, ##STR5## --N.dbd.CH--, --OCH.sub.2 --, or a single bond; R is an alkyl or alkoxy group of 1.about.18 carbon atom; Y is an alkyl, alkoxy, alkoxy carbonyl, alkanoyl or alkanoyloxy group, each having an asymmetrical carbon atom, and the other chiral smectic liquid crystal compound having a twist sense of helix left-handed is expressed by the general formula (I) and does not form a racemate. (3) A ferroelectric chiral smectic liquid crystal composition having a helical pitch of 2 .mu.m or more, wherein a chiral smectic liquid crystal compound having a twist sense of helix right-handed is an optically active compound in which the side chain Y in the general formula (I) described in the above-mentioned item (2) is ##STR6## derived from (S)-2-methylbutanol and a chiral smectic liquid crystal compound having a twist sense of helix left-handed is an optically active compound in which the side chain Y of the general formula described in the formula (I) of the above-mentioned item (2) is ##STR7## derived from (S)-2-methylbutanol. (4) A ferroelectric chiral smectic liquid crystal composition having a helical pitch of 2 .mu.m or greater wherein a chiral smectic liquid crystal compound having a twist sense of helix right-handed is an optically active compound in which the side chain Y of the general formula (I) described in the above-mentioned item (2) is ##STR8## derived from (S)-2-methylbutanol and a chiral smectic liquid crystal compound having a twist sense of helix left-handed is an optically active compound in which the side chain Y in the general formula (I) described in the above-mentioned item (2) is ##STR9## derived from (S)-1-methylheptanol (5) A ferroelectric chiral smectic liquid crystal composition having a helical pitch of 2 .mu.m or greater wherein a chiral smectic liquid crystal compound having a twist sense of helix right-handed, is an optically active compound in which the side chain Y of the general formula (I) described in the above-mentioned item (2) is ##STR10## derived from (S)-1-methylheptanol and a chiral smectic liquid crystal compound having a twist sense of helix left-handed is an optically active compound in which the side chain Y of the formula (I) described in the above-mentioned item (2) is the ##STR11## group.
The second aspect of the present invention lies in (6) a light switching element which is characterized in the use of a ferroelectric chiral smectic liquid crystal composition comprising, respectively, one or more kinds of chiral smectic liquid crystal compound having a twist sense of helix right-handed and one or more kinds of chiral smectic liquid crystal compound having a twist sense of helix left-handed.